Applicants' apparatus and methods relate to devices and methods for mixing and distributing liquid descending in exchange columns for heat and/or mass transfer processes. The apparatus and methods have particular application in cryogenic air separation processes utilizing distillation, although the apparatus and methods also may be used in other heat and/or mass transfer processes that use packing (e.g., random or structured packing). Applicants' methods also relate to methods for assembling devices for mixing and distributing liquid descending in exchange columns.
As used herein, the term “column” (or “exchange column”) means a distillation or fractionation column i.e., a column where liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture, such as by contacting of the vapor and liquid phases on packing elements (in a “packed column”) or on a series of vertically-spaced trays or plates mounted within the column.
Cryogenic separation of air is carried out in distillation columns wherein liquid and vapor mixtures are brought into intimate contact with each other. In each distillation column a vapor phase of the mixture ascends with an ever increasing concentration of the more volatile components (e.g., nitrogen) while a liquid phase of the mixture descends with an ever increasing concentration of the less volatile components (e.g., oxygen). Various means, such as packings or trays, may be used to bring the liquid and vapor phases of the mixture into contact to accomplish mass transfer between the phases.
There are many process cycles for the cryogenic separation of air into its main components, namely nitrogen, oxygen, and argon. A typical process known as the double column cycle is shown schematically in FIG. 1. Only the distillation columns and the associated cryogenic heat exchangers are shown in this schematic illustration for brevity. The double column cycle includes a high pressure column 10, a low pressure column 12, an argon column 14, a main reboiler condenser 16, a sub-cooler 18, an argon condenser 20, and an argon condenser can 22.
High pressure feed air 24 at about 4.5-5.5 bara pressure and near its dew point is fed into the base of the high pressure column 10. Within the high pressure column 10, the air is separated into nitrogen-enriched vapor 26 and oxygen-enriched liquid 28. The oxygen-enriched liquid 28 is subcooled in the sub-cooler 18, let down in pressure to about 1.2-1.4 bara and fed into the argon condenser can 22. The nitrogen-enriched vapor 26 is passed into the main reboiler-condenser 16 where it is condensed against boiling oxygen, which provides boilup to the low pressure column 12. The condensed nitrogen-enriched liquid 30 is partly used as reflux 32 for the high pressure column 10 and partly used as reflux 34 for the low pressure column 12 after subcooling the latter in sub-cooler 18 and letting it down in pressure to about 1.2-1.4 bara. In the low pressure column 12, the various feeds are separated by cryogenic distillation into oxygen-rich and nitrogen-rich components.
Gaseous oxygen product 35, also known as GOX, is withdrawn from the bottom of the low pressure column 12 and gaseous nitrogen product 36, also known as LPGAN, is withdrawn from the top of the low pressure column 12 and warmed through the sub-cooler 18 before being fed to other parts of the plant. A waste stream 38 is also withdrawn from an intermediate location in the low pressure column 12, warmed through the sub-cooler 18 and fed to other parts of the plant.
A vapor phase side-stream 40 is withdrawn from another intermediate location in the low pressure column 12 and fed to the bottom of the argon column 14 where, after flowing up the argon column 14 and condensing, it returns as a liquid stream 42, which is fed back into the low pressure column 12. The refrigeration for the argon condenser 20 is provided by partial evaporation of stream 28 in the argon condenser can 22, from where it is fed partly as vapor 46 and partly as liquid 48 to the low pressure column. Part of the vapor at the top of the argon column 14 is withdrawn as crude argon 50, also known as CGAR, and fed to other parts of the plant for further processing.
In each of the distillation columns, separation is accomplished in one or more sections, such as section 11 in the high pressure column 10, sections 13, 15, 17, 19, and 21 in the low pressure column 12, and sections 23 and 25 in the argon column 14. While different types of contact means, such as trays or packing, may be used, in the Examples discussed by Applicants, the contact means are all assumed to be made of structured packing and are shown as such.
There are many devices for distributing liquid flow uniformly over a packed section of a packed column. Such devices are disclosed in various patents and textbooks. For example, there are trough-style distributors with multiple parallel regions wherein liquid collects and flows through an array of perforations to the packing below. The liquid troughs may be laid out uniformly over the column cross section and may be fed by a central channel that runs perpendicular to the troughs and which itself may cover a major portion of the column diameter. In large columns, the liquid troughs also may be connected near the wall through an annular gutter, which may be a means for equalizing hydraulic gradients. Vapor flows in parallel regions in between the liquid troughs in generally rectangular risers. The vapor regions may have caps placed over them to prevent the liquid from the section above from falling through them and instead to channel the liquid into the troughs which collect and convey the liquid to the column section below. Such distributors may be referred to as chimney style distributors. There also are pan style distributors wherein vapor risers may be generally circular in cross section and the liquid flows around them and through perforations to the packing below. Thus, there are many basic designs and numerous variations on the basic design.
Initial presentation of liquid and vapor to the packing in a column is made by such distributors. A liquid distributor, the role of which is to irrigate the packing substantially uniformly with liquid, is located above the packing, while a vapor distributor, the role of which is to create substantially uniform vapor flow below the packing, is located below the packing. In addition to the vapor distributor, a liquid collector also is located below the packing, the role of which is to collect the liquid leaving the packing and direct the liquid further down the column. It is common for the liquid collector and the vapor distributor to be encompassed in the same device, which performs both roles.
Traditionally, there have only been a few distributors that have dealt with mixing, either independently or in combination with liquid flow distribution. These devices do not perform well in terms of their liquid composition and flow distribution quality, especially for large cryogenic distillation columns of an air separation plant having diameters greater than about 2 meters, because the devices do not provide for complete mixing (but rather only partial mixing) and uniform distribution of liquid to a packed column to achieve high separation efficiency, as do Applicants' apparatus and method. The reliability of packed columns also is improved by Applicants' apparatus and methods, which protect the packed columns from manufacturing debris and operating debris by using filters.
Various types of devices have been used in packed columns or mass transfer columns, where such devices perform, at least in part, one or more of the following functions with respect to liquid descending in a column: collection, distribution, redistribution, and mixing. However, for various reasons, these devices do not completely mix the liquid and therefore do not provide uniformity in both flow and composition of the liquid. For example, such devices are disclosed in U.S. Pat. No. 5,158,713 (Ghelfi, et al.); U.S. Pat. No. 5,776,316 (Potthoff, et al.); U.S. Pat. No. 5,752,538 (Billingham, et al.); U.S. Pat. No. 5,935,389 (Hine, et al.); and U.S. Pat. No. 7,114,709 (Ender, et al.). See also the liquid collector and redistributor disclosed in U.S. Pat. App. Pub. No. US 2009/0049864 (Kovak, et al.). Other examples of such devices include the devices disclosed in U.S. Pat. No. 5,240,652 (Taylor, et al.); U.S. Pat. No. 5,897,748 (Kaibel); U.S. Pat. No. 6,086,055 (Armstrong, et al.); and U.S. Pat. No. 7,007,932 (Armstrong, et al.).